<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
<script>
    function Foo() {
    var i = 0;
    return function() {
        console.log(i++);
    }
}

var f1 = Foo(),
    f2 = Foo();
f1();
f1();
f2();

</script>
</body>
</html>